Methods

Special Projects

Data

Acknowledgements:

These data are made freely available to the public and the scientific
community in the belief that their wide dissemination will lead to
greater understanding and new scientific insights. The availability of
these data does not constitute publication of the data. NOAA relies on
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represented.
Every effort is made to produce the most accurate and precise
measurements possible. However, we reserve the right to make
corrections to the data based on recalibration of standard gases or for
other reasons deemed scientifically justified. We are not responsible
for results and conclusions based on use of these data without regard
to this warning.

Outlier flags:

Due to limited resources each aircraft profile of greenhouse gases
is limited to one sample at each altitude which means that there are
no duplicate samples to assess the precision of each
measurement. While sampling and measurement errors can often be
identified by the lack of pressure in a flask; low flow rates at the
time of sampling or analysis or an acknowledged error at the time of
sampling or analysis, there are often less definitive indicators of
sampling or measurement errors that need to be noted. Based on our
experience with the data we have created a second column flag that
effectively scores each data point based on four factors; taken by
themselves these factors may not indicate a sampling or measurement
error, but when taken together may suggest errors. The four factors
can be listed from least to most important.

Other carbon cycle gases were outliers – The least important
indicator is based on the idea that because other carbon cycle gases
(CO2, CO, CH4 , SF 6, N2 O and H 2- CCG) are
outside the normal variability observed for that site it is possible
that the gas of interest (GOI) could be questionable. If this is the
case we assign the GOI a 1 to the second column flag to indicate that
other CCG gases from that same flask are “outliers”.

Gas of interest is an outlier – If the GOI is more than 3
standard deviations away from a function that fits the trend and the
estimated seasonal cycle, an “O” (outlier) is added to the third
column. If no other gases from that flask are flagged as outliers a 2
is assigned to the second column flag.

Aircraft cabin contaminant concentrations are high –
With more than an order of magnitude increase in
concentration of Benzene, C
2H
2(Acetylene),
and Halon-1211 (which collectively we will call HATS gases) in an
aircraft cabin, these gases have become very important indicators of
leaks in the sampling systems (PCP, 12-pack, inlet etc.). If any of
these HATS gases have been flagged with a second column "N",
indicating a suspected, but not confirmed, leak, we assign a 3 to the
second column flag in the gas of interest.

Suspect sampling error – The sampling log
and first column "N" or "L" flags in HATS or other CCG gases
(indicating a sampling problem, often due to a leak in the system) are
believed to be a good indication that all trace gas measurements from
that flask or set of flasks from a 12-pack are not valid. These
samples are given a 6 in the second column suggesting that they should
also be considered for a first column flag if they do not have one already.

Using these four factors a flagging system has been developed to indicate
the potential for error in each sample. A numerical second column flag is
automatically generated by software based on the criteria described above
and summarized in the table below. A sample with a value of 6 or higher is
re-examined individually to determine whether a first column flag is appropriate.
If it is determined that that sample has been contaminated, a first column flag of "S" is assigned.